In my prior U.S. Pat. Nos. 3,717,827 and 3,815,137 issued on Feb. 20, 1973 and June 4, 1974 respectively, as well as in prior U.S. Pat. No. 3,124,768 issued Mar. 10, 1964, interference problems in the field of radio communication are discussed. Briefly these problems involve the simultaneous utilization of one antenna or transmission line with two or more transmitting and receiving pieces of equipment operating at carrier signals of different frequencies such as are found in multicouplers in general and in diplexers and duplexers specifically. My prior co-pending applications Ser. No. 826,412 filed Aug. 22, 1977 and Ser. No. 952,011 filed Oct. 20, 1978 are also concerned with and are directed to the design of filters and multicouplers assembled therefrom.
In order to properly isolate various pieces of equipment from one another, a number of filter networks are commonly utilized as is taught in the multicoupler of U.S. Pat. No. 3,124,768. Each such network includes a first cavity resonator and a quarter wavelength transmission line tuned to pass only the frequency of the signaling device connected to the network, and a second cavity resonator and a second quarter wavelength transmission line tuned to block only the frequency of the signaling device and to pass the frequencies of the other signaling devices. Each of the second cavity resonators and second transmission lines are connected in series and in turn are connected to the common antenna.
While the multicoupler taught in patent 3,124,768 is suitable for many applications, it nevertheless poses difficulties which have not heretofore been easily and inexpensively solved. A first difficulty of the prior art devices is that the arrangement of cavity filters and quarter wavelength transmission lines required to act as transformers, require friction couplings to electrically join the cavity filters, the transmission lines, and other components into a unified system. It is well known that friction couplings create intermodulation interference problems: the greater the number of friction couplings, the greater the intermodulation interference. Additionally, it is well recognized that transmission lines introduce insertion losses which may detrimentally reduce signal strength. Since the prior art device taught in U.S. Pat. No. 3,124,768 requires a multiplicity of quarter wavelength transmission lines and a multiplicity of friction connectors, both intermodulation interference and insertion loss problems are present.
Thus, it is evident that an improved multicoupler with reduced numbers of required transmission lines and friction couplings is needed to reduce to a minimum the intermodulation interference loss problems of the prior art devices. Obviously, a multicoupler having smaller numbers of these components will also have the advantage of being significantly less expensive.
Typical prior known multicouplers utilize standard cavity bandpass and notch filters as the resonating components in their networks. A standard notch cavity filter includes an electrically resonant cavity with a moveable co-axial electrically conducting center probe for tuning the resonant frequency and a coupling loop connected at one end to the transmission line and grounded at its opposite end on the interior of the cavity. In a multicoupler, the standard notch filter acts as a short circuit in the transmission line spaced off a quarter wave from the junction at which the high impedance is desired. Varying the position, length, profile, etc., of the coupling loop permits the inductive coupling between the cavity and the transmission line to be increased or decreased. Such variation of the inductive coupling increases or decreases the loading of the cavity and hence increases or decreases the attenuation produced by the notch of the filter.
While such adjustability is desirable, standard prior art notch cavity filters have the deficiency that variation of the notch depth by adjustment of the grounded electrical loop causes the resonant frequency of the cavity to shift. When the notch of the notch filter shifts in this manner, it detrimentally effects the performance of the multicoupler. Accordingly, if one wishes to vary the attenuation of the reject band of the notch filter of prior art multicouplers, not only would the inductive coupling between the grounded coupling loop and the cavity have to be adjusted, but also the resonant frequency of the cavity itself would have to be adjusted so as to shift it back to the frequency of the respective signaling device.
Accordingly, in many prior art applications in which notch filters have been used, adjustment of the filters to increase or decrease frequency isolation has involved a complicated readjustment of not only the inductive coupling with the cavity but also of the cavity resonant frequency. Conversely, adjustment of a typical prior art notch filter to tune it to a different frequency has required a dual adjustment of tuning the resonant frequency of the cavity and then varying the inductive coupling of the grounded loop of the cavity so as to compensate for the effect produced on the depth of the notch by the change in resonant frequency of the cavity.
It is evident therefore that a filter having notch depth tuning characteristics and frequency tuning characteristics independent of one another is desirable and would be especially useful in the context of a multicoupler. With such a filter, the multicoupler could be adjusted and tuned in a variety of ways without involving a complicated interdependent fine tuning operation. It is also desirable that such a filter, when properly connected with other components in a multicoupler, perform the functions of both a notch filter and a bandpass filter at different frequencies so that the number of cavity filters required for proper operation of the multicoupler may be reduced to a minimum.